Our studies indicate that at least one common thymic differentiation antigen may be synthesized only during the brief period, early in thymocyte maturation, when the cells proliferate rapidly. Other cortical thymocyte markers continue to be synthesized long after the cells have stopped dividing. We plan to examine whether this differential gene regulation represents an irreversible developmental transition, signaled by unique factors, or simply a cell cycle-dependent pattern of expression. The spectrum of products made by the proliferating thymocytes will be compared with that of small thymocytes restimulated by mitogens. Partially purified T lymphocyte growth factors will be assayed for stimulatory, inhibitory or stabilizing effects on the synthesis of each of these gene products in different populations of thymocytes. The mechanisms of phenotypic change will be probed by monitoring changes in the pools of the mRNAs that encode these products, as revealed by in vitro translation. Finally, to correlate these biochemical events with function, the T cell growth factors will be assayed for possible protective effects on thymocytes exposed to glucocorticoids to determine any role they may play in the manifestation of the "mature" cortisone-resistant thymocyte phenotype.